Various applications require the formation of a seal between adjacent components such that the seal prevents the flow of fluids between the components. In some cases, the seals are configured to completely prevent the flow of fluid, while in other cases it is desirable to have a small amount of fluid flow through the seal. Further, some applications require a complete seal at certain times and a partial seal at other times so that the flow of the fluid can be selectively controlled. For example, FIG. 1 illustrates a conventional turbopump 10 for a rocket engine, such as the high pressure fuel turbopump for the space shuttle main engine available from the Rocketdyne division of The Boeing Company. The turbopump 10 includes a pump portion 12 and a turbine portion 16. A shaft 20, sometimes referred to as a “rotor,” extends between the two portions 12, 16 to mechanically couple a pump 14 in the pump portion 12 to a turbine 18 in the turbine portion 16, so that the pump 14 can be rotatably actuated by the turbine 18.
During operation, the pump 14 is used to pump cold fluids such as liquid hydrogen or liquid oxygen, and thus the pump portion 12 typically operates at cryogenic temperatures. The dimensions of, and clearances between, components of the turbopump 10 can be affected by the temperature of the turbopump 10. Therefore, the pump portion 12 is typically chilled before operation of the engine is started. Such chilling can be accomplished by circulating a cryogenic fluid through the pump portion 12. The turbine portion 16, on the other hand, typically operates at high temperatures, e.g., 1000° F. or greater. It is generally undesirable to chill the turbine portion 16 before operation. Moreover, if the cryogenic fluid is combustible, it is generally undesirable for the fluid to leak into the turbine, which can increase a risk of fire. Therefore one or more seals 22, 24 are provided for preventing the cryogenic fluid from flowing from the pump portion 12 to the turbine portion 16 during the pre-operation chilling process. However, during operation of the turbopump 10, it is generally desirable for some of the fluid to leak from the pump portion 12, through the seals 22, 24, and into the turbine portion 16 so that the fluid cools the turbine portion 16.
The turbopump 10 shown in FIG. 1 includes seals 22, 24 for controlling the flow of the cryogenic fluid. In particular, a lift-off seal 22 and a labyrinth seal 24 are provided at various locations in the turbopump 10. Selective sealing between the pump and turbine portions 12, 16 of the turbopump 10 is achieved by the lift-off seal 22, which is actuated by the pressure of the cryogenic fluid. Typically, the lift-off seal 22, which extends circumferentially around the shaft 20, is disposed adjacent a shoulder 28 provided on the shaft 20 or on a sleeve 30 that is on the shaft 20. The lift-off seal 22 includes springs that bias a sealing surface of the lift-off seal 22 against the shoulder 28. When the pressure in the pump portion 12 is low, such as during the pre-operation chilling process, the springs maintain the sealing surface against the shoulder 28, thereby preventing the flow of fluid between the seal 22 and the shoulder 28. When the pressure in the pump portion 12 increases, such as during operation of the turbopump 10, the fluid overcomes the springs in the lift-off seal 22, pushing the sealing surface away from the shoulder 28 so that fluid can flow therebetween. However, due to the number of parts and complexity of the lift-off seal 22, such seals can fail and improperly prevent or provide fluid flow between the pump and turbine portions 12, 16. Failure of the seal 22 can result in improper or inefficient operation of the turbopump 10 or failure of the turbopump 10, thereby requiring maintenance, repair, or replacement of the turbopump components. Further, lift-off seals 22 are typically relatively large, requiring significant space in the turbopump 10.
Thus, there exists a need for an improved sealing assembly for turbopumps and other applications requiring a fluid seal. The sealing assembly should preferably provide a controllable seal so that a flow of fluid through the seal can be prevented according to a particular application or operational aspects of the application. In addition, the sealing assembly should preferably be reliable and not unnecessarily large or complex.